The tracks on an optical recording medium such as a compact disc (CD) or digital versatile disc (DVD), form a long spiral. In order to read and/or write data from/to the recording medium, a disc driver must position its optical head or optical pickup unit (OPU) onto a designated data point (the corresponding track) and a laser beam spot must follow the track on the rotating disc. An OPU includes a light source (for example, laser diode), an optical system, including an objective lens, to focus the laser beam onto the optical disc, and a photo-detector array to detect the reflected/diffracted light from the disc.
The laser beam from the OPU includes a primary beam and two secondary side beams provided on both sides of the primary beam. The primary beam is to be centered on a track (pit area) and detected by a main photo-detector (quadrant detector). The secondary beams are to be centered on the “land” formed both sides of the track, and detected by two additional (side) photo-detectors place alongside of the main detector. If the primary beam is right on the center of the track, the two sides of the main photo-detector and the side photo-detectors would (ideally) receive equal amount of light from the secondary beams. As the optical head drift to either side of the track, the amount of light reflected from the tracking beams varies. Diffraction would cause a change in the light intensity on the two different sides of the photo detector array. The difference signal (push-pull signal) is proportional to the amount of deviation of the beam from the center of the track.
The movement of the OPU is controlled by actuators. A typical servo control of a disc driver includes focus servo control and tracking servo control. The tracking servo could be in rough (coarse) seek, fine seek, or tracking mode. The rough seek is performed by a sled actuator or “sled” which leads the optical head to the vicinity of destination along the surface of the disc radially, and thus this operation is also referred to as “sled seek.” The terms “sled seek” and “rough seek” are used interchangeably in this specification. The fine seek is performed by a fine actuator located inside the optical head, which drives the lens within the housing of the optical head so as to precisely and rapidly position the laser beam on to the desired track while keeping the tracking servo loop closed. The fine seek is typically performed within 1,000 tracks.
As the disc rotates, the laser beam follows the spiral of the track from the inner diameter (ID) to the outer diameter (OD). However, if the disc has run out due to manufacturing tolerance or otherwise, the center of the spiral may not be the center of rotation of the spiral. Thus, as the disc rotates, the track moves radially with respect to a fixed point, for example, the laser beam spot. This radial motion is referred to as “run out.” The repeatable run out (RRO) error is a repeatable, predicable off-track motion caused by a mis-centering of a disc on the spindle motor, i.e., a physical misalignment between the center of the disc and the center of the rotation. Typically, the RRO is less than 150 μm for DVDs, and less than 280 μm for CDs.
FIGS. 1A through 1D schematically illustrates the run out seen at the rotational angles 0, π/2, π, and 3π/2, respectively. In FIGS. 1A through 1D, the solid line 10 represents the track position with run out, and the broken line 12 represents the track position without run out. In order to reduce the effect of this run out error and other errors in laser beam positioning (or “tracking”), a closed loop control system is used. While the closed loop control is operating, the lens will move radially along with the disc so that the laser beam spot is held on the center of the track.
The “track position” is defined as location of the laser beam spot with respect to the track center, which is detected by imaging the beam spot onto a photo-detector array, as mentioned above. Diffraction causes a slight change in intensity of the light received on the two sides photo-detectors. This difference in the intensity, i.e., the push-pull signal, is proportional to the track error (TE).
However, if the laser beam from the light source does not travel directly through the center of the optical lens, i.e., the laser beam is off-centered, reflection and refraction of the light would shift the image of the beam spot to one side of the photo-detector array. This image-shifting effect due to the off-centered laser position on the lens is referred to as the “center error” or “CE”. The center error is also observed as the difference in the light intensity received by the two sides of the photo-detectors, and the push-pull effect of the center error and that of the track error are not distinguishable by examining one track position alone. Thus, in order to obtain an accurate track error, a second measurement is taken one-half track away. The effect of the center error is the same in the first and second measurements, but the push-pull effect of the track error is reversed in the first and second measurements. By combining these two measurements an accurate track position (track error) can be determined. For example, the push-pull signals of the first measurement (main push-pull: MPP) and the second measurement (side push-pull: SPP) are expressed as follows:
      MPP    =          TE      +      CE            SPP    =                  1        k            ⁢              (                              -            TE                    +          CE                )            Where k is a factor added because the second measurement receives less light. Thus, the net track error TE is obtained as:TE=MPP−k×SPP 
As discussed above, in order to position the laser beam spot onto the center of the track on the rotating disc, the lens moves radially once per revolution, following the radial disc movement due to the run out. That is, when the laser beam is locked to the disc, the lens radially moves with respect to the optical center of the OPU. Here, the optical center is the center of the housing of the OPU, along which the center of the laser beam is aligned. This lens motion induces a center error that is synchronous to the disc rotation.
When it is desired to move the laser beam spot from a track to another track, either the lens by itself can be moved within the housing of the OPU (fine seeking), or the OPU including the lens can be moved by the sled actuator (rough or sled seeking). The fine seeking is fast because the laser beam spot stays locked to the disc, even while track crossing. However, if the target track is far away, the lens cannot move far enough within the OPU. Thus, the sled is used to reposition the OPU to the vicinity of the target track. During the sled seek, the laser beam spot is unlocked from the disc, and the center error is used to control the laser beam spot to stay at the center of the housing of the OPU. This is done in order to prevent the lens from inadvertently hitting the housing when the sled accelerates or decelerates the OPU, since such an impact can cause the lens to lose focus.
When the laser beam spot is locked to the track, a closed loop control system of the disc driver eliminates the effect of run out. However, during a rough seek (or sled seek), as described above, the laser beam spot is unlocked from the track. Thus, upon completion of the rough seek, the laser beam spot must be locked to the disc again. However, if the run out is large, the relative motion between the laser beam spot and the disc may also be large, making it difficult to lock the laser beam spot to the disc.